1. Field of the Invention
The present invention relates to an image processing apparatus such as an electronic camera or the like which is configured so as to electronically record image information, and particularly to an image processing apparatus in a memory control system in which the output data from a solid state image sensor is stored in a DRAM.
2. Description of the Related Art
A conventional electronic camera corresponding to a television system (NTSC or PAL) frequently uses SRAM as memory for storing the data of a solid state image sensor because of high access speed. The SRAM has the disadvantage that a memory capacity which is only enough for one image can be contained in the camera body because of the small capacity and high cost of the SRAM. However, the SRAM has advantages with respect to simple control of writes and reads, rapid access, and a relatively short time required for transferring data because the data capacity handled in the camera is small, as described above. The execution of writes and reads to and from memory and the memory address management accompanying the execution can be controlled directly by a microprocessor, without the need for a special memory control system. However, when a recent HDTV system (High Definition Television) is used, the memory capacity must be increased as the amount of the pixel data for one image is increased, and reads, writes and transfers of the data must be performed at high speed. It is preferable from the viewpoint of a balance of cost and packaging area to use a general purpose DRAM memory for an electronic camera which requires a large amount of data.
However, when DRAM is used as memory, read from a solid state image sensor must be temporarily (refresh cycle) stopped for a refresh operation of the DRAM. The time taken from charge storage in the solid state image sensor to data read therefrom is increased by the stop time, thereby generating much dark current and causing noise spots on the sensor.
The control of the DRAM is also complicated, and the processing speed thereof is lower than that of the SRAM. In addition, the refresh operation is required for holding the stored contents, conditions for address setting and write and read timing are severe, and the access speed is low. A system is thus required for appropriately controlling the DRAM while relating it to the solid state image sensor and other peripheral devices.
A solid state image sensor for color images generally has color filters having a plurality of colors (RGB or YMC) which are bonded to the surface thereof in a mosaic or stripe form so that a color is represented by combination of output of a plurality of pixels corresponding to the filters. If the output of a pixel is saturated, therefore, the color of the periphery of the pixel cannot be correctly reproduced. However, when a general object is photographed, the output of the pixels in a portion or the whole of the image plane of the solid state image sensor are sometimes saturated because the object is excessively bright.
If the output from a pixel is greater than a predetermined value, it is generally decided that the pixel is saturated. In reproduction, the gain (GAIN) of the color of the periphery of the pixel is thus decreased by signal processing for preventing the occurrence of a false color.
However, when the dark current of the solid state image sensor is decreased for decreasing the noise thereof on the basis of the image pickup data of the object, as in a conventional element, since the dark currents of the respective pixels are nonuniform, the level of a pixel which is saturated is decreased due to subtraction. As a result, it is impossible to discriminate an unsaturated pixel and a saturated pixel. There is thus the problem of difficulty in processing for suppressing a false color of a high-brightness portion.
The decrease in dark current also causes the following problem: When general recorded data is reproduced to an image, a high noise component is decreased by passing through a filter. In this case, it is assumed that there is a dark current noise, as shown in FIG. 14(b) below. In FIG. 14, the horizontal position is shown on the abscissa, and the amplitude is shown on the ordinate. In this case, the result of subtraction of the dark current as shown in FIG. 14(b) from original data as shown in FIG. 14(a) is as shown in FIG. 14(c) . The resultant data is passed through a filter to produce data as shown in FIG. 14(d). There is thus the problem that the noise cannot completely be removed.